Appointments legacy time calculus ignores padding

Our site has multiple providers and multiple services. Some of the services have a 15 minute padding afterwards.

We have found that if we set the time slot calculus to "Minimum time based appointment duration calculus (legacy)," it ignores the padding and allows appointments to be scheduled immediately after the previous appointment.

We were avoiding the problem by setting the time slot calculus to "Service duration based calculus." But now we have noticed that available start times are changing depending on the duration of the appointment being scheduled.

According to this forum post we understand that we should change to the legacy time calculus. How can we do that and still keep the service padding?